This invention relates generally to the field of materials technology, and more specifically to the field of abradable coatings, and in particular to a process for manufacturing a turbine component by applying an abradable coating using a cold spray technique, and to a turbine component manufactured with such a process.
Abradable coatings are well known in the art. An abradable coating may be applied to a component that is subject to rubbing or abrasion during the operation of the component. The abradable coating is selected to be softer than the material of the underlying component and the material of the rubbing structure. As a result of its mechanical properties, the abradable coating will wear preferentially in lieu of the wearing of the underlying material or the rubbing structure. By purposefully causing an interference fit between the two structures, the abradable coating will be caused to wear to a minimum clearance fit, thereby acting as a seal between the two structures.
It is known that the efficiency of a turbine engine depends to a large degree upon minimizing the leakage of the working fluid from a desired flow path. As used herein, the term turbine may include any type of aero-rotary machine, such as a steam turbine, combustion turbine, compressor, etc.
Primary sources of such working fluid leakage are the clearances between moving and stationary parts within a turbine. Although a close tolerance fit may be obtained by fabricating the mating parts to a very close tolerance range, such fabrication processes are very costly and time consuming. Furthermore, as an engine cycles through its speed and power ranges it will experience temperature transients that can result in a temporary change in dimensions, and with very tight tolerances can result in an unplanned contact between moving and stationary parts. Abradable coatings have become an industry standard for controlling the size of such clearances. The function of such a coating is to provide a rub-tolerant surface that minimizes the damage to the rubbing parts, and can thereby permit the nominal gap between such parts to be minimized.
It is known to apply an abradable coating to the inner diameter surface of a compressor blade ring forming part of a gas turbine engine. The abradable coating is much softer than the material of the compressor blade tips, therefore, any interference between the blade tips and the blade ring will result in the preferential wearing of the abradable coating, concomitantly establishing a blade tip seal. The substrate material of a compressor blade ring is typically a carbon steel. A common abradable coating for this application is nickel-graphite, such as 85% nickel 15% graphite by weight. The carbon in this material acts as a lubricant during the wearing of the abradable coating surface. A bond coat is necessary between the carbon steel material of the blade ring and the coating of abradable material to prevent the corrosion of the underlying carbon steel. Because abradable coatings are by design somewhat porous, they will allow moisture and other corrosive materials to migrate into contact with the carbon steel. Any corrosion caused by such exposure of the carbon steel can cause spalling of the abradable coating.
To apply such an abradable coating to a compressor blade ring, it is known to clean the substrate surface to remove any corrosion or oxidation products. Such cleaning may be accomplished by grit blasting with alumina particles. A nickel-aluminum bond coat, typically 5% by weight aluminum, is then applied to the cleaned carbon steel substrate. The bond coat may be applied by any one of several thermal spray processes, including flame spray, air plasma spray (APS) and high velocity oxy-fuel (HVOF). Such processes propel the bond coat material in a molten or semi-molten state against the surface of the substrate where it cools and solidifies to form a coating. Although it is desirable to completely seal the surface of the carbon steel with the bond coating layer, such thermal spray processes often produce a coating having some porosity. The abradable coating is then applied to the bond coat material, again by a thermal spraying process. Care must be taken when applying the bond coat layer and the abradable material layer to prevent the warping or ovalization of the blade ring due to differential heating/cooling of the component.
The known processes for applying abradable coatings have numerous limitations, such as the creation of coating layers containing voids and porosity, the need for specialized thermal spraying equipment that is not easily adaptable for field repair operations, and a high cost of manufacturing. For certain components such as a blade ring, the high temperature of the thermal spraying process can cause distortion of the component. Thus, an improved process is needed for manufacturing components having an abradable coating.
The present inventors have recognized that a cold spray process is beneficial for the application of an abradable coating system. The cold spraying of the bond coat layer of an abradable coating system provides an oxidation and corrosion resistant coating having less porosity than prior art bond coat layers. Because a cold spray process produces a bond coating having essentially no porosity, the performance of the overlying abradable coating will be improved when compared to prior art flame sprayed coatings because the incidence of spalling will be reduced.
For components sensitive to warping or distortion, the use of a cold spraying process for both the bond coat layer and the abradable material layer eliminates any concern of heat induced deformation.
Because the area to which a coating is applied may be limited and controlled during a cold spraying process, an abradable coating may be applied to only a selected area of a component without the need for masking of the areas not to be coated.
The porosity of a layer of abradable material may be controlled to a desired value by controlling the parameters of a cold spray process.
The halo effect of particles along the edges of a cold spray of particles provides a final cleaning of the surface to be coated. This final cleaning is especially beneficial when coating a carbon steel component, since even a small amount of oxidation forming after an initial grit blasting process will reduce the adhesion of an overlying bond coat. The use of a cold spray process not only provides this final cleaning action, but it also eliminates the oxidation effects of a thermal spray process.
A portion of the carbon in a nickel graphite abradable material will be oxidized during a thermal spraying process and will escape as carbon monoxide or carbon dioxide gas. An abradable coating produced by cold spraying a nickel graphite powder will contain more of the beneficial carbon than a similar coating produced by thermally spraying the same powder. Since no loss of carbon occurs during cold spray, very precise control of composition is achieved.
These and other features and advantages of the invention are provided by way of example, not limitation, and are described more fully below.